Fastener tension control system

ABSTRACT

A method and apparatus for controlling final tension on a fastener being run up and set. An efficiency factor is developed and utilized for attaining predetermined final tension on a fastener, or a group of similar fasteners, being operated upon. The efficiency factor is a ratio between work on a theoretical fastener, i.e., one wherein friction load is absent in work calculations, and work on an actual fastener. Having obtained the efficiency factor, the actual torque load on a fastener to attain predetermined tension value, can be established by use of the efficiency factor.

BACKGROUND OF THE INVENTION

This invention relates to apparatus and a method for controlling torqueapplied to a fastener to achieve a predetermined final tension load onthe fastener, or on each fastener of a group of like fasteners beingsimultaneously operated upon, and to constantly monitor torque andefficiency on each fastener of the group to assure that final tensionload is maintained thereon until final tension load is developed on allfasteners of the group.

New and improved methods, or systems, are constantly being developed andutilized in the fastener tightening art. Most methods currentlyemployed, operate to control torque to achieve desired ultimate fastenertension. In assembly operations, involving multiple fasteners, torquecontrol monitoring permits the assembly to be checked to assure that allfasteners being operated upon attain predetermined final tension valuebefore torque loading is terminated. However, the value of tension on agiven fastener in an assembly operation depends upon certain variablessuch as friction and load in the assembly.

For any given fastener there is a constant relationship between fastenerelongation, fastener tension or load, and fastener rotation relative toits fixed threaded counterpart, which relationship is not affected byfriction. Some of the known fastener tightening systems attempt toutilize this relationship to control fastener tension. However, themeans used to apply this relationship are affected by friction. Examplesof frictional effects in known tightening systems are:

1. The known systems assume that the torque-angle curve is astraight-line, for the cycle portion where all rotation is absorbed byelongating the fastener. This straightline is used to establish afastener's zero load point, from which the tightening angle is measured.If the frictional effects are not constant the straight-line assumptionis incorrect and the fastener will not be tightened correctly.

2. Some of the known systems use the yield point of the torque-angletightening curve to correlate tightening angle with bolt tension.However, yield point of a fastener, as developed on an actualtorque-angle curve, is a result of stresses produced from fastenertension load as well as friction.

Another deficiency of the systems which control fastener rotation toachieve final tension, is that there is no way to check a completedassembly to determine if it has been tightened to the correct angle ofrotation to obtain a tension specification.

The subject invention incorporates development of a theoreticaltorque-angle curve which is used to establish calculation of anefficiency factor. Such a curve is not used in known systems of theprior art. The converting of actual torque to fastener tension throughsuch an efficiency factor and a fastener constant, is more accurate thanrelating an actual fastener yield point to bolt tension. Locating atheoretical curve with respect to the actual torque angle curve issimpler and more accurate than projecting the shape of the actual curveback to final zero load angle and then rotating to a predesignated angleto achieve desired final torque.

Development and use of the efficiency as disclosed herein allowsmeasurement of fastener torque with a torque wrench the value of which,when multiplied by efficiency and a fastener constant, will determinethe clamp load of the fastener under consideration. Such will allow acheck of the system itself to determine if desired fastener tension hasbeen developed, or can be later used to tighten a fastener to the torquerequired to achieve desired level of clamp load.

SUMMARY OF THE INVENTION

This invention utilizes a novel method and apparatus to accomplish themethod whereby final tension load on a fastener can be accuratelycontrolled, whether a single fastener is being operated upon, or aplurality of like fasteners in an assembly operation. By use of anefficiency factor which can be calculated, establishment of atheoretical torque angle curve can be made, whereby torque loading toachieve final tension in the fastener can be established.

It is a further feature of the invention that the disclosed system willmeasure the effect of friction during a tightening cycle and determinewhat torque is required to obtain the desired tension in the fastener,or fasteners being operated upon.

Another feature of the invention is that the system enables checking tosee if desired tension loading is achieved, or to later determine whattension loading was achieved on a fastener.

These and further features and objects of the invention will bedisclosed hereinafter including certain charts and diagrams asillustrated in the drawings wherein:

FIG. 1 is a graph illustrating an actual vs. theoretical tighteningcurve;

FIG. 2 is a graph illustrating development of a formula for area underan actual torque-angle curve;

FIG. 3 is a graph illustrating established actual, as well astheoretical torque-angle curves, as used to determine an efficiencyfactor;

FIG. 4 is a schematic showing apparatus and circuitry to accomplish thesystem of the invention in set up phase; and

FIG. 5 is the same but showing operation of the system to accomplish afastener tightening operation.

DESCRIPTION OF THE INVENTION

Referring to the curve showing an actual torque vs. angle curve as wellas a theoretical curve (FIG. 1), the theoretical curve is a torque vs.angle curve without friction. The torque at yield (ty) is proportionalto the fastener tension load at yield. The angle (αy) is proportional tothe amount of fastener elongation that occurred from no load to yield.Both of these values (αy) and (ty) can be predetermined for any size andworking length of fastener. The slope of the theoretical curve is ty/αyand will be referred to as (C₁). The equation for the theoretical curveis then: t=C₁ α

Therefore, for an assembly where the portion of final tightening goesonly into the elongation of the fastener the theoretical torque anglecurve is t=C₁ α.

The set points Ti and Tm will now be selected. Ti must be great enoughto ensure the parts of the assembly are clamped and Tm must be lowenough to ensure the fastener will not yield at this value. The effectof friction is measured by efficiency, which is the actual value of workrequired, divided into which the value of work required to tighten afastener if friction was not present. The area under the torque anglecurves is proportional to the work or energy required to increase thefastener torque from Ti to Tm. The area under the actual curve from zerotorque is Ti θc+A1 and likewise the area under the theoretical curve isti θc+a1, for the θc segment of the curves (FIG. 3).

Therefore the efficiency is: ##EQU1## Efficiency is also equal to thetheoretical torque divided by the actual. Thus

    Eff.=ti/Ti

or

    Ti=ti/Eff.

Substituting Ti=ti/Eff in the above equation gives ##EQU2##

Therefore, only the area between Ti and Tm and ti and tm needs to becalculated to determine efficiency. Curve of FIG. 2 shows that the areaunder the actual curve A1 is: A1=Δθ[T₁ "+T₂ "+T₃ " . . . +1/2TM"].Likewise the curve of FIG. 3 shows the area under the theoretical curveis

    a.sub.1 =(1/2)c.sub.1 θc.sup.2.

The actual area A1 and theoretical area a1 under their respective curvesare determined and the efficiency calculated for each tightening cycle.Once the efficiency is known then the theoretical torque value (ti) isdetermined by multiplying the values of set point Ti and efficiency.Since the equation for the theoretical curve is known, the determinationof one torque point on the curve, permits the determination of thetorque at any other point. The torque (tm) which is equal to, the torqueat (ti) plus the slope multiplied by the angle between ti and tm, ortm=ti+c₁ θc where c₁ is the slope of value ty/αy.

The theoretical torque (ta) multiplied by fastener constant is equal tothe desired tension or fastener load. Fastener constant refers to axialdisplacement per angular rotation, which is fixed for any givenfastener, or fasteners having the same structural detail i.e.,proportions, and is equal to fastener pitch radius multiplied by thelead angle tangent. This theoretical torque (ta) will be obtained whenthe theoretical torque (tm) is added to the slope (C₁) multiplied by theangle (β) turned beyond (tm) or:

    ta=tm+c.sub.1 β.

If the efficiency remained constant, the actual torque value (Ta)required for ta would be Ta=ta/tm (Tm) (FIG. 1).

Since the efficiency may change, the actual torque Ta may not give thetheoretical value ta. Thus when the actual torque equals Ta, this valueof torque is held on the fastener, but the theoretical torque value (t)is checked to see if it equals (ta). If it does not, a new value ofactual torque (T_(f)) is predicted based on the new efficiency and theprocess is repeated. When the theoretical torque (t) equals (ta), theactual torque required to obtain this is held on the fastener. Theefficiency at this point is determined by dividing the theoreticaltorque (t) by the actual (T_(f)) and displayed. The operator can nowcheck the fastener for the desired tension, since he knows the torque,efficiency, and fastener constant it took to obtain this load.

The foregoing is the procedure followed when the slope C₁ can bepre-determined by measuring the tension and elongation that occurred atfastener yield. In the other type of assembly where the latter portionof the tightening cycle consists of assembly part deflection andfastener elongation, the slope must be determined on the completeassembly, since the angle rotation is absorbed by fastener elongationand the assembly. Such is done as follows:

Please refer to curve shown in FIG. 1. The equation of the theoreticaltorque is still t=C₁ α, but in this case C₁ is unknown. To determine C₁,the value of theoretical torque (ty') at the yield point of the actualtorque angle curve must be used. This value must be an average valuesince it does vary somewhat with friction, but it is a value that othertension systems use to control load. The value of angle (θy) from noload to yield will be a different value than (αy). The illustrationshows θy less than (αy), but depending on how much deflection occurs inthe assembly, it could be larger than (αy).

The slope C₁ can be determined in the following manner: ##EQU3##

The set points Ti and Tm are selected as hereinbefore described. A setup fastener is then run to yield and A₁, θc and α₂ obtained. The tensionsystem uses these values in the above equation and solves for C₁. OnceC₁ is determined by the set up fastener, the subsequent fasteners arerun accordingly.

Operation of the Subject System

A schematic of the system is illustrated by circuit arrangements shownin FIGS. 4 and 5. FIG. 4 shows the system in set up and FIG. 5 shows itcontrolling a tightening cycle.

As seen in FIG. 5, air is fed to a recovery type nut runner (5) such asdisclosed in U.S. Pat. No. 3,507,173, granted Apr. 21, 1970. A pressureregulator (3) sets a rundown pressure on a pilot controlled regulator(4). This rundown pressure is a value that causes the fastener to betightened to a torque exceeding Tm. The nut runner (5) then holds thistorque on the fastener.

An angle encoder (6) drives a clock circuit (7) which gives one clockpulse per-degree of fastener rotation in the tightening direction, orcan be set for a pulse each two degrees, or whatever degree setting isdesired. A torque transducer (8) gives an analog voltage signal that isproportional to the torque being exerted on the fastener. A peak torqueelement (10) holds the last highest value of torque that passed throughit. The torque value is fed to a comparator (11) where nothing happensuntil the torque value exceeds set point Ti (12).

The first portion of the circuit functions as a fastener yield detectorused for emergency shut-off to prevent fastener breakage, and to preventany angular rotation that occurs without building torque from enteringthe control circuit.

This circuit operates as follows:

A clock pulse moves a torque value from the comparator (11) holds it ina sample and hold element (13). The same clock pulse moved the previoustorque value that was in the sample and hold element (13) to a sampleand hold element (14). A differential amplifier (15) takes thedifference between these two values and feeds it to a comparator (16).If the difference value is less than a set point value (17), then inpasses to a pulse counter (18). If the difference values betweensuccessive pulses remain below the set limit (17) for enough pulses toexceed a set limit (21), then the air to the nut runner (5) will be shutoff by valve (1). This would be a rejected tightening cycle.

When the difference between successive pulses is greater than the setlimit (17) it goes to a single pulse circuit (20) which resets the pulsecounter (18). If the torque is less than a set value (56) then the pulsegoes on to a pulse count circuit (22). The pulse count circuit (22)totalizes the pulses that are above the limit (17) until the torquebecomes equal to a set point (56). Thus the element (22) contains thenumber of pulses or degrees between the torque points Ti and Tm which isthe angle θc shown on curve (FIG. 1). This value of pulses is fed to aD-A converter (23) that converts the count to an electric voltage andstores it in a sample and hold element (24). The portion of the circuit(25) thru an adder (31) determines the area (A₁) under the actual torquecurve per the equation A=Δθ [T₁ "+T₂ "+T₃ " . . . +1/2Tm"] see FIG. 2.The torque value from the sample and hold element (13) is fed to adifferential amplifier (25), that subtracts the setpoint value Ti (12)from it. The difference value is then fed to a sample and hold (26). Thenext clock pulse moves it to an analog adder (27) where it is added tothe previous pulse values from the sample and hold (28). An AND logicelement (55) is provided to prevent the clocking of the sample and holdelements (26) and (28) when the difference between torque pulses is lessthan the set point (17). The total of these values are held in thesample and hold (29). When the torque exceeds a set value Tm (56) thenext pulse above the set point (17) passes through a single pulsecircuit (32). This circuit puts out a clock pulse which moves the valuein the sample and hold (29) to the adder (31). It is then added to a setpoint value (30). The output of adder (31) is the value of A₁ shown onthe curve of FIG. 2.

The clock pulse from circuit (32) also moves the value of θc stored inthe sample and hold (24) to a multiplier (33), where it is multiplied bya set point 1/2₁. This value then goes to another multiplier (34) whereit is multiplied by θc again. The output of this multiplier is C₁ θc² /2or a₁ per the equation on curve of FIG. 3.

The values A₁ from adder (31) and a₁ from the multiplier (34) are fed toa divider (35). The output of divider (35) is the Eff. A multiplier (36)multiplies the set point Ti (12) by the Eff. to obtain the theoreticaltorque ti. The theoretical torque value from (36) is added by an adder(38) to the output C₁ θ_(c) from multiplier (37). The output of adder(38) is theoretical torque value tm. This value tm is fed to an adder(39) to be added to the value (C₁ β) from a multiplier (41). The valueC₁ β is the value of the angle past Tm multiplied by the slope constantC₁. At Tm this value is zero, thus the output of adder (39) is still tm.The value of tm then goes to a divider (43) where it is divided into thetheoretical torque value ta from a set point (42). The value ta is thetheoretical torque needed to obtain desired clamp load. The clock pulsefrom (32), when the torque exceeded Tm, passed through an OR logicelement (54) and clocked the value of torque on the fastener at Tm tomultiplier (44). The output from a divider (43) ta/t is multiplied bytorque from a sample and hold (47). This is the value of actual torqueTa that will give the theoretical torque value ta if the efficiencyremains constant. Value Ta from (44) is fed to a differential amplifier(45). When the actual torque equals or exceeds Ta, a voltage will begiven from the amplifier (45). This signal goes to valve (2) which stopsthe pressure to the nut runner from increasing. Thus this torque valuewill be held on the fastener. The signal from amplifier (45) also passedthrough a single pulse circuit (46). This circuit (46) produces a clockpulse that goes to a comparator (51). If the value ta/tm from divider(43) is greater than set point (50), the clock pulse will pass through alogic element (57) to sample and hold (40) and through (54) to (47).When the sample and hold (40) is clocked, a value of β is fed tomultiplier (41). The output of (41) is added to (tm) at the adder (39).This value is divided into (ta) by the divider (43). The clock pulse atsample and hold (47) clocks the present value of torque on the fastenerand said value is sent to the multiplier (44). The output of themultiplier (44) is now the new torque predicted to obtain thetheoretical torque value (ta). If the new predicted torque is greaterthan the torque now on the fastener, the output of amplifier (45) willstop and valve (2) will reopen and the pressure to the tool willincrease.

This cycle will repeat until the theoretical torque (t) equals desiredpercentage of (ta). At this point the actual torque will equal thepredicted torque and valve (2) will be closed, holding this torque valueon the fastener. The clock pulse from circuit (46) will pass through thecomparator (51) to the sample and hold (52), since the value ta/t willbe less than set point (50).

The clock pulse at (52) will clock the value of the efficiency fromdivider (48) to display (53). The cycle is now complete.

Please refer to the schematic in FIG. 4 for the operational descriptionof the system in set-up. The value "one" is put in for C₁ and the valuety', from curve of FIG. 1 is put in for ta. A tightening cycle will berun on the actual assembly. The yield detecting circuit, items (13) thru(19) will terminate the tightening cycle at yield. The value C₁ will bedetermined by the equation ##EQU4## When the torque exceeds Ti, items(25) thru (31) calculate the area under the actual torque curve A₁, asexplained in the tightening cycle. Once the torque exceeds Tm set point(56) the circuit (32) gives out a single clock pulse. This pulse clocksthe value θc from the sample and hold (24) to the multiplier (33). SinceC₁ is now one, θc is multiplied by 1/2. This value 1/2θc goes tomultiplier (34) where it is multiplied again by θc. The output ofmultiplier (34) is now θc ² /2. This value goes to the divider (35)where it is divided by A₁ from the adder (31). The output from thedivider (35) is fed to the multiplier (36), where it is multiplied by Tiset point (12). The output of (36) is now Ti θc² /2A₁. The value 1/2θcfrom the multiplier (33) also goes to multiplier (37) where it becomesθc. This value θc is now added to the output value of multiplier (36) byadder (38). The output of adder (38) is Ti θc² /2A₁ +θc. When the yielddetecting circuit gives the signal from the comparator (19) to close thevalve (1), this signal also goes to the OR logic element (57). Thesignal passes through the logic element (57) and clocks the sample andhold (40). This moves the value of the angle from Tm to yield (β) to themultiplier (41). Since a value of one was put in for C₁ the output ofmultiplier (41) is still (β). This value goes to adder (39) to be addedto the output of adder (38). The output of adder (39) is now Ti θc²/2A+θc+β. The divider (43) now divides this value into ty' from setpoint (42). The output of divider (43) is the value C₁ and is displayedin item (49).

The value C₁ and ta will now be put in their set-point elements. Thesystem can now be used to control the tightening cycle as illustrated inFIG. 5.

What is claimed is:
 1. A tension control tightening method wherein amotor is powered to run down and set a fastener and means are providedto regulate power to the motor to obtain a predetermined tension set onthe fastener, said method comprising the steps of:initiating operationof the motor for turning the fastener; generating a signal indicative offastener rotations; generating a signal proportional to the torque beingexerted upon the fastener by the rotation means; utilizing both of saidsignals to determine the work being expended on the fastener; comparingsaid work to equivalent work on a theoretical fastener having nofriction load; establishing an efficiency factor based on saidcomparisons; utilizing said efficiency factor to determine actual torquerequired to obtain predetermined tension on the fastener; and cuttingoff power to the motor when said predetermined tension is obtained onthe fastener.
 2. In a tension control tightening method according toclaim 1, wherein the method is being employed to simultaneously run downand set a plurality of similar fasteners, the steps of:maintainingpredetermined tension in each fastener until said predetermined tensionis obtained on all of said fasteners; and cutting off power to allfastener motors when said predetermined tension is obtained on all thefasteners.
 3. In a tension control tightening method according to claim1, wherein said efficiency factor is the ratio between the area under atheoretical torque rotation curve between first and second rotationpoints on the theoretical curve, and the area under an actual torquerotation curve between said two rotation points on the actual curve. 4.In a tension control tightening method according to claim 3, whereintheoretical torque multiplied by a fastener constant equals desiredfastener tension.
 5. In a tension control tightening system according toclaim 4, wherein the torque exerted on the fastener at the firstrotation point is great enough to ensure that the parts of an assemblybeing held by the fastener are fully clamped together, and the torqueexerted on the fastener at the second rotation point is low enough toensure that the fastener yield point is not exceeded.
 6. Apparatus foruse in a tension control tightening method for running down and settinga fastener comprising a recovery type nut runner:means to supplyoperative power to said nut runner; means to measure torque beingexerted by the nut runner on the fastener; means to develop a signalwhich indicates rotation on the fastener caused by the nut runner; meansto develop an analog signal which is proportional to torque beingexerted by the nut runner on the fastener; means to utilize both signalsto determine work being expended on the fastener by the nut runner;means to compare said work to equivalent work on a theoretical fastenerhaving no friction load; means to establish an efficiency factor basedon comparisons carried out by the comparing means; means to utilize saidefficiency factor to determine actual torque required to obtainpredetermined tension on the fastener; and means to cut off saidoperative power to the nut runner when said actual torque is developedon the fastener.
 7. The apparatus of claim 6, wherein a plurality ofsaid nut runners are utilized for simultaneously running down aplurality of similar fasteners to obtain said predetermined tension ineach of said fasteners; andmeans to cut off operative power to all ofsaid nut runners when said predetermined tension is developed on each ofsaid plurality of fasteners.